The present invention relates to a silver halide photographic light-sensitive material, and more particularly, to a lithographic silver halide photographic light-sensitive material (referred to simply as the "lith sensitive material" hereinafter).
A lith sensitive material comprises a high contrast, silver halide photographic emulsion coated on a support, and can provide a high contrast silver image composed of dots and lines by receiving a development-processing using an extremely high contrast developer such as to have gamma (.gamma.) of more than 10 (referred to as a lith developer hereinafter), as is described in detail, for example, in J.A.C. Yule, Journal of the Franklin Institute, volume 239, page 221 (1945). The thus-obtained, high contrast silver image can be employed as a photomaster for printing.
However, it frequently happens that lith development-processing as described above results in a reddish-brown silver image (referred to as "colored fringe" hereinafter). It is thought that this phenomenon results from the generation of fine colloidal silver particles around silver image and the scattering of light from these particles. These fine colloidal particles are thought to be generated at a stage of development wherein the developed silver has not yet grown to its full size, and therefore has not yet formed a filamentary structure. For instance, the extent of change in color of the silver image, from black to reddish-brown, becomes particularly large when a large amount of polyalkylene oxide compound is incorporated in a lith sensitive material or a lith developer for the purpose of raising image quality, when an exhausted lith developer is used, or when a lith developer is contaminated with even only a small amount of fixing solution. Such colored fringe around silver image is undesirable for a photomaster in which exact reproduction of halftone dot (image area) and line width is required. Therefore, it is strongly desired to prevent colored fringe from occurring.
In general, compounds having thione or mercapto groups, as described, for example, in Japanese Pat. No. 13485/68, C.E.K. Mees and T. H. James, The Theory of Photographic Process, 3rd edition, pages 325-328, McMillan, New York (1966), etc., are known to render the color tone of a silver image pure black (referred to as "cool black toning"). Since such compounds were found to exhibit the cool black toning effect in cases that developers of metol-hydroquinone system and developers of phenidone-hydroquinone system were used, it may be analogized that these compounds can be useful for the apparent improvement upon color tone of silver image in developers of other systems also.
As will be described hereinafter, thiones (e.g., 3,4-methyl-thiazoline-2-thione) can undoubtedly impart a cool black tone to a silver image, but suffer the serious disadvantage that they cause a lowering of sharpness in the toe portion of the characteristic curve obtained using a lith developer, and, consequently, cause deterioration of qualities of halftone dots and line images.
On the other hand, mercapto compounds, for example, 1-phenyl-5-mercaptotetrazole and the like, which are well-known in the above-described literatures and so on, can not only impart a cool black tone to the silver image, but can also improve upon the sharpness of the toe portion and thereby, raise the quality of halftone dots. However, they also suffer two serious disadvantages that they increase the frequency of occurrence of black pepper (also known as pepper spots) and cause considerable desensitization when the lith developer used is contaminated by a slight amount of fixing solution by accident.
The term "black pepper" signifies black spots which appear in areas among dots, which areas are normally to be the non-developed part. Such black spots appear in large numbers when the lith developer is close to becoming exhausted, or when the lith developer is contaminated by only a slight amount of fixing solution and thereby, the commodity value as a lith sensitive material is greatly lowered. Further concerning the black pepper, a detailed description thereof appears in H. Zwicky, Journal of the Photographic Science, Volume 23, page 159 (1975), and in other papers, but the cause of its occurrence remains unexplained in many respects.
Moreover, it happens that a sharp decrease in the sensitivity is observed in some lith sensitive materials processed using an automatic developing machine. This is, as will be shown hereinafter, supposed to be caused when the lith developer is contaminated by a slight amount of fixing solution through the splash of the fixing solution, a mistake made while working, the passage of processed films (in which a fixing solution remains unremoved in only a trace amount) through the lith developer with the intention of removing pollution from the rack of the developing machine, or so on.
As described above, the compounds reported in the aforementioned art have some effects upon cool black toning, but produce undesirable side effects in the lith development-processing. Some of them spoil the effect of lith development and, therefore, cannot be used in lithographic materials.